Phenylacetic acid derivatives of the formula I where the substituents and the index have the following meanings: X is oxygen or sulfur; R is hydrogen or alkyl; R1 is hydrogen or alkyl; R2 is cyano, nitro, trifluoromethyl, halogen, alkyl or alkoxy; m is 0, 1 or 2, it being possible for the R2 radicals to be different if m is 2; R3 is hydrogen, cyano, nitro, hydroxyl, amino, halogen, alkyl, haloalkyl, alkoxy, haloalkoxy, alkylthio, alkylamino or dialkylamino; R4 is hydrogen, cyano, nitro, hydroxyl, amino, halogen, unsubstituted or substituted alkyl, alkoxy, alkylthio, alkylamino, dialkylamino, alkenyl, alkenyloxy, alkenylthio, alkenylamino, N-alkenyl-N-alkylamino, alkynyl, alkynyloxy, alkynylthio, alkynylamino, N-alkynyl-N-alkylamino; unsubstituted or substituted cycloalkyl, cycloalkoxy, cycloalkylthio, cycloalkylamino, N-cycloalkyl-N-alkylamino, cycloalkenyl, cycloalkenyloxy, cycloalkenylthio, cycloalkenylamino, N-cycloalkenyl-N-alkylamino, heterocyclyl, heterocyclyloxy, heterocyclylthio, heterocyclylamino, N-heterocyclyl-N-alkylamino, aryl, aryloxy, arylthio, arylamino, N-aryl-N-alkylamino, hetaryl, hetaryloxy, hetarylthio, hetaryl amino, N-hetaryl-N-alkylamino; R5 is hydrogen, unsubstituted or substituted alkyl, cycloalkyl, alkenyl, alkynyl, alkylcarbonyl, alkenylcarbonyl, alkynylcarbonyl or alkylsulfonyl; unsubstituted or substituted aryl, arylcarbonyl, arylsulfonyl; hetaryl, hetarylcarbonyl or hetarylsulfonyl, and their salts, processes and intermediates for their preparation, and their use are described.